Method and system for precisely positioning a waist of a material-processing laser beam to process microstructures within a laser-processing site

ABSTRACT

A high-speed method and system for precisely positioning a waist of a material-processing laser beam to dynamically compensate for local variations in height of microstructures located on a plurality of objects spaced apart within a laser-processing site are provided. In the preferred embodiment, the microstructures are a plurality of conductive lines formed on a plurality of memory dice of a semiconductor wafer. The system includes a focusing lens subsystem for focusing a laser beam along an optical axis substantially orthogonal to a plane, an x-y stage for moving the wafer in the plane, and a first air bearing sled for moving the focusing lens subsystem along the optical axis.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is related to provisional patent application entitled“Trajectory Generation And Link Optimization”, filed the same day as thepresent application. Also, this application is related to U.S. patentapplications entitled “Precision Positioning Apparatus” filed on Sep.18, 1998 and having U.S. Ser. No. 09/156,895, and “Energy EfficientLaser-Based Method and System for Processing Target Material” filed onDec. 28, 1999 and having U.S. Ser. No. 09/473,926.

TECHNICAL FIELD

This invention generally relates to methods and systems for high speedlaser processing (machining, cutting, ablating) microstructures. Morespecifically, this invention relates to methods and systems forprecisely positioning a waist of a material-processing laser beam toprocess microstructures within a laser-processing site. Semiconductormemory repair is a specific application where precise positioning indepth of the beam waist of the laser beam is required to dynamicallycompensate for local variations in height of the wafer or targetsurface.

BACKGROUND ART

Memory Repair is a process used in the manufacture of memory integratedcircuits (DRAM or SRAM) to improve the manufacturing yield. Memory chipsare manufactured with extra rows and columns of memory cells. Duringtesting of the memory chips (while still in the wafer form), any defectsfound are noted in a database. Wafers that have defective die can berepaired by severing links with a pulsed laser. Systems generallyutilize wafer-handling equipment that transports semiconductor wafers tothe laser process machine, and obtain the information in the form of anassociated database specifying where the links should be cut andperforms the requisite link ablation for each wafer.

Successive generations of DRAM exploit finer device geometry in order topack more memory into smaller die. This manufacture of smaller devicesaffects the geometry of the links allocated for laser redundancy. As thedevices get smaller, the links get smaller and the pitch (link-to-linkspacing) shrinks as well. Smaller link geometry requires a smaller spotsize from the laser in order to successfully remove selected linkswithout affecting adjacent links, preferably with little if anycompromise in throughput.

All systems focus the laser-processing beam to perform memory repair andrequire that the surface of the link be maintained within a smalltolerance of the beam waist (focus) position with depth. When the linkis in the focal plane of the lens, the focused spot will be minimumsize. At focus or “beam waist height” above or below nominal, the spotwill be defocused with the magnitude of defocus increasing with distancefrom nominal. A defocused spot reduces the energy that is delivered tothe target link possibly leading to insufficient cutting of the link. Adefocused spot may also place more laser energy on adjacent links or onthe intervening substrate leading to possible substrate damage. At somelevel of defocus, the laser cutting process is no longer viable.

The allowable tolerance for relative placement of the lens and link isreferred to as “depth of focus” (DOF). The depth of focus criteria is afunction of the process tolerance for the particular link and lasercombination. Experiments are typically performed over a range ofoperating parameters, including focus height, in order to determine thesensitivity of the laser cutting process to the parameters. Forinstance, from these experiments it might be found that the laser wouldreliably sever links when the combinations may exhibit more or lessprocess latitude to focus height.

Prior generation memory repair systems perform a focus operation onceper site. As more dies are processed within a single site, the sitedimensions get larger. This presents a problem in that the wafers seldomare flat (planar) and parallel to the focal plane. If focus is performedat only one point within a site, then the system will operate slightlyout of focus at points within the site that are not near to the focuslocation.

At least three factors affect the ability of a memory repair system tomaintain the link in focus.

-   -   1. The process or sensor used to measure focus may exhibit        errors.    -   2. The wafer may exhibit “topology” that requires different        focus heights at different locations over the surface of the        wafer.    -   3. The mechanism used to provide relative motion between the        wafer surface and focal plane may exhibit errors.

A process for compensating height variations was used in 1992 by apredecessor company of the assignee of the present invention (i.e.“GSI”) to perform thin-film trimming on integrated circuits (IC) innon-wafer form. At the time, IC's were being packaged into sensors andthen trimmed after packaging. The problem encountered at the time wasdue to the packaged die being significantly non-parallel to thesurrounding package (typically pressure sensors). Incorporating aZ-Roll-Pitch mechanism for positioning the device in the product solvedthe problem at the time. An auto-collimator sensor was included in theoptical path and used to measure the angle of the die surface relativeto the focal plane. The angular information from the auto-collimator wascombined with a single focus measurement to define a plane. Themechanism then moved the die in 3 axes to place the die into thebest-fit plane compensating for Z, roll and pitch. The range of dietilting was sufficiently large that it was often necessary to performiterative corrections to properly focus the die. After making anadjustment in Z, roll and pitch, a second set of focus and tiltmeasurements was made followed by a subsequent (smaller) focus and tiltcorrection.

One problem of this approach is that the auto-collimator worked bestwhen it could be directed at a large “planar” object. With pressuresensors, it was often possible to define a large region that lackedsurface features in order to use as an auto-collimator target. It wouldnot be possible to find such a region on a typical IC found in memoryrepair applications.

In 1994, GSI developed a different approach to handle thin-film trimmingon “tilted die.” The problem was again due to trimming on packaged IC(pressure sensors). In this case, the specifics of the customer's deviceprecluded the use of a tilting Z-stage. A single Z-axis stage was usedin the product and the Z-stage was moved in coordination with X and Ypositioning of the laser beam. Also, the absence of suitable targetstructures for the auto-collimator on certain customer's devices forcedGSI to develop the multi-site focus algorithm. Height measurements wereobtained using a sensor that obtained a sequence of measurements alongthe z-axis from which the position of best focus was correlated tosurface position—a prior art method known as “depth from focus”. Theprocess was repeated at 3 non-collinear locations. A best-fit plane(exact in the case of 3 points) was used to coordinate the movement ofthe device that was mounted to the Z-stage.

Prior art laser-based, dynamic focus techniques and/or associated “depthfrom focus” are widely used over a range of scales and at variousoperating speeds. Exemplary systems operating at a microscopic scale aredisclosed in U.S. Pat. Nos. 5,690,785, 4,710,908, 5,783,814, and5,594,235, and selected pages of Chapter 7 entitled “Optics for DataStorage” in the book “Laser Beam Scanning” by Marcel Dekker, Inc., 1985.A desirable improvement in the memory processing or the processing ofother microstructures would provide capability to generate and applyindustry-leading small spot sizes to the applications with improvedthroughput. In turn, an improved figure of merit for resolution andspeed in the presence of local depth variations which substantiallyexceed the DOF associated with the small spot sizes.

DISCLOSURE OF INVENTION

An object of the present invention is to provide a high-speed method andsystem for precisely positioning a waist of a material-processing laserbeam to process microstructures within a laser-processing site.

It is an object of the invention to provide a method and system forhigh-speed laser processing of microstructures on a surface havingthree-dimensional coordinates wherein the surface has substantial localwarpage, wedge, or other variations in depth. The variations introduce arequirement for high speed, 3-dimensional relative motion of the targetand laser beam, within a die site for example, so as to dynamically andaccurately position the beam waist. The beam waist, which may be lessthan 1 um in depth, is to substantially coincide with the 3D location ofthe microstructure.

It is an object of the invention to provide an improved“speed-resolution product” in 3 dimensions—where the control system forthe wafer movement preferably provides movement in 2 directions, and thehigh precision lens actuator provides beam focusing (e.g.: positioningof the beam waist) action in the third dimension.

It is an object of the invention to reduce the alignment time of theprocess by estimating a surface which is used to define a trajectorythereby eliminating or minimizing any requirement for die-by-diealignment or die-by-die focus measurements.

It is an object of the invention to maintain the correct focus height(i.e.: beam waist position) over the entire site (i.e.: several dice ofa wafer), thereby improving over prior art focus solutions capable ofonly maintaining focus at locations adjacent to the focus location.

It is an object of the invention is to process dice on a wafer with asingle “site.” The field size may allow as many as six or eight 64 MDRAM dice to be processed at one time. This process is called multi-diealign (MDA). The use of MDA affords a significant throughput improvementby reducing the number of alignment operations required to process thewafer from 1-per die to 1-per site. The prior art alignment operationsmay require roughly the same amount of time to perform as link cuttingfor a single die.

In carrying out the above objects and other objects of the presentinvention, a method for precisely positioning a waist of amaterial-processing laser beam to dynamically compensate for localvariations in height of microstructures located on a plurality ofobjects spaced apart within a laser-processing site is provided. Themethod includes providing reference data which represents 3-D locationsof microstructures to be processed within the site, positioning thewaist of the laser beam along an optical axis based on the referencedata, and positioning the objects in a plane based on the reference dataso that the waist of the laser beam substantially coincides with the 3-Dlocations of the microstructures within the site.

The objects may be semiconductor dice of a semiconductor wafer whereinthe microstructures are conductive metal lines of the dice.

The objects may be semiconductor memory devices.

The step of providing may include the step of measuring height of thesemiconductor wafer at a plurality of locations about the site to obtainreference height data. The step of providing may further include thesteps of computing a reference surface based on the reference heightdata and generating trajectories for the wafer and the waist of thelaser beam based on the reference surface.

The reference surface may be planar or non-planar.

The method may further include varying size of the waist of the laserbeam about the optical axis.

The step of providing may include the steps of reducing power of thematerial-processing laser beam to obtain a probe laser beam andutilizing the probe laser beam to perform the step of measuring.

Further in carrying out the above objects and other objects of thepresent invention, a system for precisely positioning a waist of amaterial-processing laser beam to dynamically compensate for localvariations in height of microstructures located on a plurality ofobjects spaced apart within a laser-processing site is provided. Thesystem includes a focusing lens subsystem for focusing a laser beamalong an optical axis, a first actuator for moving the objects in aplane, and a second actuator for moving the focusing lens subsystemalong the optical axis. The system further includes a first controllerfor controlling the first actuator based on reference data whichrepresents 3-D locations of microstructures to be processed within thesite, and a second controller for controlling of the second actuatoralso based on the reference data. The first and second actuatorscontrollably move the objects and the focusing lens subsystem,respectively, to precisely position the waist of the laser beam and theobjects so that the waist substantially coincides with the 3-D locationsof the microstructures within the site.

A support supports the second actuator and the focusing lens subsystemfor movement along the optical axis.

The system may further include a spot size lens subsystem forcontrolling size of the waist of the laser beam, a third actuator formoving the spot size lens subsystem wherein the support also supportsthe spot size lens subsystem and the third actuator for movement alongthe optical axis, and a third controller for controlling the thirdactuator.

The first actuator may be an x-y stage.

The second and third actuators may be air bearing sleds for supportingthe focusing lens subsystem and the spot size lens subsystem,respectively, both mounted for sliding movement on the support.

A voice coil is coupled to its respective controller for positioning itsair bearing sled along the optical axis.

The system may further include a position sensor such as a capacitivefeedback sensor for sensing position of the focusing lens subsystem andproviding a position feedback signal to the second controller.

The laser beam may be a Gaussian laser beam.

The system may further include a trajectory planner coupled to the firstand second controllers for generating trajectories for the wafer and thewaist of the laser beam. At least one of the trajectories may have anacceleration/deceleration profile.

The system may further include a modulator for reducing power of thematerial-processing laser beam to obtain a probe laser beam to measureheight of the semiconductor wafer at a plurality of locations about thesite to obtain reference height data. The system may include a computerfor computing a reference surface based on the reference height datawherein the trajectory planner generates the trajectories based on thereference surface which may be planar or non-planar.

The invention improves upon the prior art by including two steps:

-   -   1. Height measurements are performed at multiple (typically 4 or        more) points surrounding the die site.    -   2. The focus (beam waist) height is adjusted as the laser beam        is positioned within the site so as to maintain best focus        throughout the site based on fitting a surface to multiple        height measurements.

A method for high speed laser processing of micro-structures havingthree dimensional coordinates includes the steps of:

Selecting a plurality of reference locations on a surface from whichheight data is to be obtained, obtaining height coordinates at theplurality of reference locations separate from but in proximity tomicro-structures, estimating three dimensional locations ofmicro-structures from the coordinates of the reference locations,generating a trajectory adapted to position micro-structures relative toa location defining a laser processing beam axis, determining theposition of an optical component disposed in path of the laserprocessing beam such that the corresponding position of the beam waistof the focused laser processing beam will substantially coincide with acoordinate of a micro-structure when the micro-structure is positionedto intersect the active laser processing beam, inducing relativemovement between micro-structures and the location of a laser processingbeam while coordinating the movement of the optical element in the pathof the laser processing beam to dynamically adjust the position of thebeam waist of the processing laser beam whereby the location of the beamwaist substantially coincides with a coordinate of the micro-structurewhen it intersects the laser processing beam, providing a laserprocessing beam pulse to process the microstructure while relativemovement is occurring between the micro-structures and the laserprocessing beam.

The height information will preferably be obtained from the same laserand optical path used for processing, but with reduced power (with amodulator used to reduce the power and avoid damage to the surface).

Alternatively, a separate tool may be used to measure the height of thesurface at reference locations.

In a construction of the invention, the estimated surface location maybe computed from a planar fit, higher order surface fit, throughbilinear interpolation.

A straight line approximation may be used for micro-structures locatedin a row.

The preferred optical system has capability for both spot size selectionand focus control.

The optical focusing system is preferably mounted on an air bearingsled.

In a preferred system, spot size adjustment is provided with zoomelements mounted on an air bearing sled which independently adjusts spotsize.

In a preferred system a high precision voice coil motor is mounted tothe optical box and operatively connected to the air bearing sled.

In a preferred system, the position of the focusing optical system ismonitored with a high band width position sensor, such as a capacitivefeedback sensor.

In a preferred construction the positioning of the lens or opticalelement provides Z-axis resolution of about 0.1 um with a half powerbandwidth of about 150 Hz.

In a preferred construction of the present invention, the maximumvelocity of the wafer movement stage during processing is in the rangeof about 50-150 mm/sec.

The preferred range of movement of the optical element corresponds toabout 3 mm movement range of the beam waist along the Z direction.

In a construction of the invention, the response of the actuatorcontrolling the beam waist position can correspond to an incrementalchange in depth within a duration of about 0.03 msec.

A numerical offset may be introduced to compensate for the thickness ofoverlying passivation layers covering the micro-structure, or otheroffsets with respect to the reference surface.

The spot size at the three-dimensional coordinate of the microstructureis preferably within 10% of the diffraction limited (smallest) spot sizeafter relative movement of an optical element.

The energy enclosure at the three-dimensional coordinate of themicrostructure preferably exceeds 95% size after relative movement of anoptical element.

The peak energy of the processing laser spot will preferably exceed 90%of the maximum peak energy.

The laser beam may be substantially Gaussian and TEM00.

The z coordinate of the beam waist is preferably dynamically adjustedand follows a computed surface, such as a plane. The correspondingchange in depth between any two structures, including adjacentstructures in a row of microstructures, may exceed the Z-axis resolutionof the optical system positioner within a die.

The z coordinate of the beam waist is preferably dynamically adjustedand follows a computed surface, such as a plane. The correspondingchange in depth between any two structures, including adjacent die onthe wafer, may exceed the DOF of the laser beam.

A dimension of a microstructure may be less than the wavelength of thelaser, for example: 0.8 μm width, 6 μm length, 1 μm thickness spacedapart by about 1.5 μm-3 μm from center-to-center.

The tolerable DOF of the laser beam may be on the order of or less than1 wavelength of the laser processing beam.

The tolerable DOF of the laser beam may be less than 1 um.

The optical element may be moving the position of the beam waist inresponse to a continuous motion signal while the laser processing of themicrostructure is occurring.

The optical element may be moving the position of the beam waist duringthe relative motion of the laser and micro-structures.

The relative motion of the lens may be constant, or may haveacceleration/deceleration profiles provided by a trajectory planner.

A system of the present invention is able to operate with smaller spotsizes (which require better focus control) and thereby process deviceswith smaller geometry than prior memory repair systems due in part tosuperior focus control.

Dynamic Focus allows a system of the present invention to adapt to thenon-parallel and non-planar topology that is typically found on realwafers and maintain acceptable focus over the full extent of a die site.

The method and system of the present invention is to be advantageouslyapplied to semiconductor memory repair. However, it will be apparentthat the present invention is also advantageous for microscopic laserprocessing applications where the depth of focus is small compared tothe local height variations in the surface, and where the laserprocessing is to occur at high speed.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic block diagram showing a prior art system forsemiconductor memory repair;

FIG. 2 is a detailed schematic block diagram of a memory repair systemin accordance with the present invention showing the major sub-systems;

FIG. 3 is a schematic block diagram, similar to the diagram of FIG. 1,of the optical subsystem of the present invention showing theinteraction with control systems used for wafer processing, includingthe trajectory generation subsystem;

FIG. 4 is an exemplary illustration showing a wafer processing sitecomprising several die and associated regions where reference regionsare located to define a reference surface;

FIG. 5 illustrates a preferred coordinate system used fortransformations to specify the location of a laser beam relative to aprocessing site in a laser processing system utilizing a precisionpositioning system;

FIG. 6 is an illustration of the process of fitting a plane withbilinear interpolation;

FIG. 7 a is a graph showing the available depth of focus (and DOFtolerance) as a function of spot size consistent with the requirementsof link processing;

FIG. 7 b is a schematic diagram illustrating the diameter of a Gaussianlaser beam prior to, at and after its minimum spot size;

FIGS. 8 a-8 c illustrate the assembly details and operation of thehigh-speed lens positioning system used for adjusting the beam waist(focus) position between adjacent links to be processed;

FIG. 9 illustrates details of a preferred lens arrangement andpositioning system advantageous for use in practicing the presentinvention;

FIG. 10 a is a schematic view of links of dice to be processed within adie site; and

FIG. 10 b is a schematic view of motion segment types generated by thesystem of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

A preferred system of the present invention is shown in FIG. 2. A wafer4 is positioned within the laser processing system 110 and databaseinformation from the user interface 11 is provided to identify the links(33 in FIG. 4) on the wafer which are to be ablated to repair defectivememory cells.

Referring to FIG. 3, wafers exhibit wedge 28 (i.e. a plane that istilted with respect to the focal plane) and non-planar topology 281which requires compensation. The wedge 28 and local curvature 281 areexaggerated in scale for the purpose of illustration. Based upon thespecified locations of the defective cells regions are identified on thewafer (33 in FIG. 4) in reference height data is to be obtained. Suchlocations may be “bare wafer” regions that have little surface textureor other suitably defined regions that are generally selected to matchthe imaging and processing capabilities of the measurement sensor.Typically depth information will be obtained using a “depth from focus”algorithm, and may be obtained using the laser operated with lowerincident power resulting through operation of the modulator 2. Areference surface is defined from the surface height information throughmathematical techniques for surface fitting. The ideal wafer surface(one that does not require compensation) is a plane that is parallel tothe focal plane of the optical system 5, but slight non-orthogonality ofthe x, y system relative to the optical axis and/or surface variationsproduce significant height deviations 28.

Referring again to FIG. 2, the computed reference surface is used by thetrajectory planner 12 and a DSP based controller 15, 16 in conjunctionwith motion stage 6, 7, 26 calibration to define motion segments for thetrajectory generator which are executed and coordinated with laser 1,focusing optics 24, and x, y stages 6, 7 operation to ablate links. Thisoperation includes control of x, y motion with preferred high speedprecision stages 6, 7 and simultaneous positioning of optical elements24 to position the beam waist 5 of processing laser 1 to coincide with acoordinate of the link 33 when the laser is pulsed.

All memory repair systems include some dynamic mechanism to providerelative motion between the wafer surface and the focal plane. In somecases, this may involve controlling the height of the wafer 4 relativeto a fixed height optical path 3 as shown in FIG. 1 by movement 9 alongthe z-axis. Alternatively the motion 9 may utilize movement of the lens3 in a “stepwise” manner to coincide with a location in depth derivedfrom focus data from a die of site.

With the present invention the overall height of the wafer remainsconstant and the final objective lens height is controlled by a linearservo mechanism 22,23 and controller 14, 17. The positioning of the lensor optical element with a preferred arrangement using a precisionpositioning system 22-27 for Z axis movement provides Z-axis resolutionof about 0.1 um or finer with a 3 DB “small signal” bandwidth of about150 Hz, over a typical maximum range of movement of about 3 mm.

The following description provides additional detail for operation andconstruction of the preferred system of the invention. However, theembodiment and variations described below is intended to be illustrativerather than restrictive etc.

Selecting Reference Regions and Obtaining Data

A system for link blowing will generally require processing (i.e. laserablation) of a subset 33 of a large number of links 34 on a wafer. Theinformation which defines the links to be processed is provided to acontrol program. The program in turn will define a set of referencelocations 32 surrounding a number of die 35 to be processed—i.e. a“site”. The locations will generally include a sufficient number ofpoints to accurately define a trajectory to be followed by the wafer andlens system based upon commands generated for motion system control.

At each reference location height or “focus position” is measured usingan auto-focus or more precisely a “depth from focus” sensor. In apreferred system scans occur over a fixed x, y location target whilecontinually adjusting the beam waist position over a range of heights(z-axis positions). The contrast in each scan is recorded at eachheight. When the laser is in best focus, the contrast in the scan willbe maximized. Errors occur in the auto-focus routine due to randomfluctuations in the laser during the scan and mechanical vibrations inthe system. The high power processing beam may be used if the modulator2 is used to control the power delivered to the surface.

The reference information and computed surface provide exactcompensation for planar topology 28 that is either parallel ornon-parallel to the focal plane. In the case of parallel and planartopology, the dynamic focus method of the present invention is animprovement over prior art. Multiple reference sites 32 provide “bestestimates” of a surface and improved measurement and statisticalconfidence in the presence of sensor noise and mechanism errors.

For non-planar topology (ex: convex or concave surfaces), the DynamicFocus method of the present invention is an improvement, though stillnon-ideal solution. With non-planar topology 281, as illustrated inFIGS. 1 and 3, a satisfactory participation will typically be a best-fitplane. A plane can closely approximate many surface shapes as long asthe minimum radii of curvature of the actual surface is large (recognizethat a plane is a curve with infinite radius of curvature). For typicalwafer surfaces, the best-fit plane approximation is sufficient tocompensate for the majority of wafer topology with residual errors(deviation from best-fit plane) comparable to the other error sourceslisted earlier.

The dynamic focus method and system of the present invention can be usedin a variety of modes based on the form of topology that is assumed.

Mode 0: Dynamic Focus will function similar to typical prior focussystems when operated in Mode 0. A single focus measurement is made atone location, for example a die within a die-site, and a parallel planeis assumed for all locations within the site. Dynamic Focus offers noadvantage over conventional solutions when operated in this manner.

Mode 1: Making multiple focus measurements around the die site and usingthe average focus height as the parallel plane can make a slightimprovement over Mode 0. This is referred to as Mode 1. There would belittle reason to use this mode unless there were a good reason tobelieve that the actual wafer topology is indeed a parallel plane andthat the largest focus error to be corrected is due to focus.

Mode 2: Two non-collocated focus measurements can be used to fit atilted plane that contains the 2 measured focus heights, but isotherwise parallel to the focal plane. Specifically, a lineperpendicular to the line containing the 2 focus measurements isparallel to the focal plane. This mode is best considered as adegenerate case of the subsequent modes.

Mode 3: Three non-collinear focus measurements are the minimum numbernecessary to describe a best-fit plane for the die-site. This is asubstantial improvement over Modes 0-2 as long as wafer topology is alarger source of error than focus measurement (generally the case).

Mode 4: Refer to FIGS. 4 and 6. Four non-collocated and non-collinearfocus measurements (generally taken at the 4 corners of a die-site)provide a further improvement over Mode 3 in the presence of non-zeromeasurement error. If there is reason to believe that focus measurementcontributes more error than the best-fit plane assumption, then the 4measurements can be used to solve for the best-fit plane. In this case,the 4^(th) measurement is providing a slight bit of averaging to theother 3 measurements to help reduce errors due to focus measurement.

Referring now to FIGS. 1 and 6. If wafer topology 4 is believed to be alarger source of error than height measurement, then the 4 measurements32 can be used to construct a “twisted plane.” A twisted plane is abi-linear interpolation of the 4 height measurements. A bi-linearinterpolation produces the height measurements exactly when evaluated atthe focus reference sites and smoothly interpolates between themeasurements at all other points.

Beam waist position at a point x,y within the site is given by theequation: $\begin{matrix}{{Z = {{{Xg} \cdot {Yg} \cdot {Z00}} + {{Xf} \cdot {Yg} \cdot {Z10}} + {{Xf} \cdot {Yf} \cdot {Z11}} + {{Xg} \cdot {Yf} \cdot {Z01}}}}\quad{{wherein}\text{:}}} \\{{Xf} = \frac{X - {X0}}{{X1} - {X0}}} \\{{Xg} = {1 - {Xf}}} \\{{Yf} = \frac{Y - {Y0}}{{Y1} - {Y0}}} \\{{Yg} = {1 - {Yf}}}\end{matrix}$

It will be apparent those skilled in the art of measurement that the useof least squares fitting of additional reference points may improve theresults. Further, if advantageous, additional data may be collected andused to fit a higher order quadratic or cubic surface. For a given laserprocessing application the number of samples and the choice of referencesurface generally depend upon the maximum rate of change expected overthe region to be sampled.

Trajectory Planning and Generation

A “trajectory planner” 12 is utilized to plan the path of the wafer 4and beam waist position 5 with a motion system 6, 7, 17 and associatedDSP based controller 16. Included as part of the present invention is ahigh-speed precision actuator 22, 23 for the lens system 24, 25. Thetrajectory planner integrates information from the user interface andalignment system 11 that is used to define the position of the laserrelative to the targets (the latter typically mounted on a precisionstage, for instance, a wafer stage) in a coordinate system. From thedatabase the information is derived, resulting in a “link map”, dieselection, and other pertinent data regarding the memory repairoperation.

Those skilled in the art of motion control and estimation willappreciate the tolerance budget requirement for accuratethree-dimensional positioning in a high-speed memory repair processingsystem. A fraction of a micron, over a maximum range of travel of about300 mm or more, corresponds to the entire area of a modern wafer.Processing or “cutting speeds” exceeding 50 mm/sec. are advantageous.Also, the above-noted copending utility patent application entitled“Precision Positioning Apparatus,” (now allowed) and incorporated byreference describes details of a preferred wafer positioning system.

In a preferred embodiment of the present invention trajectory generationwill include a relative coordinate system organized as illustrated inFIG. 5 into “world” 41, “stage” 42, “device” 43 and “beam” 44coordinates, as illustrated by the following kinematic transformation:

-   -   _(WORLD) ^(STAGE)=_(WORLD) ^(BEAM)⊚_(STAGE) ^(DEV)⊚_(DEV)        ^(BEAM)

World. The world frame is attached to the machine base. The origin islocated at the midpoint between the two X, Y motion encoder read headsfixed to the base, and its Y axis passes through the center of each readhead.

Beam. The beam frame is attached to the optics box 18. The origin islocated at the center of the focused laser spot.

Stage. Attached to the wafer stage 6,7. It is coincident with the worldframe when the stage is at the wafer load position (lift pin holesaligned with lift pins).

Device. This frame, abbreviated as “dev”, is attached to the waferstage. Preferably beam motion is always commanded relative to the deviceframe. The device frame is arbitrarily defined by the user, to correctfor die alignment, for example. The transformation from stage to deviceframe is a six-parameter linear transform, not a rigid-body one. Forthis reason the device frame is deliberately show as a distorted frame.

The kinematic transformation relates the frames. A system definitionwhich specifies the motion at the path of the laser beam 5 with respectto a user defined device coordinate system is a convenient architectureas related to link blowing, though other applications may vary from suchan architecture. In general, the transformation T in FIG. 5 relatingstage coordinates to the device frame is a six parameter lineartransformation as opposed to a rigid body. Those skilled in the art ofmotion control, particularly as implemented in multi-axis roboticsystems, will recognize and be familiar with concepts and conventions ofreference (coordinate) frames and coordinate transforms. In a preferredimplementation a calibration step is included for all the motion stageswhich will result in data which is used in subsequent coordinatetransformations (e.g. translation, rotation, scaling) to accuratelyrelate all the coordinate systems and mathematical transforms.

In operation, beam focus position 5 is adjusted “on the fly” topreferably position the central portion (minimum width) 62 of thefocused Gaussian laser beam 5 to coincide with the link to be processed.The focus subsystem 90, which is shown in FIG. 8-9 and will be describedin more detail later, preferably includes a high performance linearvoice coil motor and associated circuitry 22, 23 mounted to the opticalsub-system. To position the beam waist the surface generated from thereference data provides the required coordinate information. In apreferred embodiment the resultant heights are constructed in the“stage” coordinate frame 42 because the “device” frame 43 will bealtered after focus correction by subsequent die alignment. However,those skilled in the art will recognize that many variations can existdependent upon specific application requirements.

Positioning the Optical Components

The precision lens system 24, 84 will be positioned so the beam waist 5,62 at the intersection of the link so that the link 33 is severedwithout damage to adjacent structures by the laser. The relativemovement may be on the order of 1 micron or finer between adjacentlinks, with peak to peak movement of perhaps tens of microns over thewafer at the preferred processing speeds. In a preferred system theoptics will provide programmable spot size control with lens system25,85 in addition to focus (beam waist) control. Those skilled in theart of laser beam manipulation will appreciate the tolerances requiredfor positioning in such a way to enclose nearly all of the laser energywithin the link area, corresponding to a small fractional loss of peakenergy at the center of the beam. FIG. 7 a illustrates the depth offocus (for specific DOF tolerances) for an ideal focused Gaussian beamlaser having a wavelength of 1.047 um. The DOF depends upon the lensnumerical aperture, laser wavelength, and is affected by beam truncationand other factors. Sub-micron tolerances are present, and the link sizesencountered in present link blowing systems are less than 1 μm in adimension, for example, 0.5 μm wide and 6 μm length on center-to-centerspacings of about 1.5-3 μm. Graph 65 is representative of requirementsfor state of the art memory repair systems. If total spot size growth ofonly 5% is required to ablate the link the total DOF is about 1 um, or±0.5 um relative to the position of best focus, for example. A standardDOF criteria (often used) of 40% is not acceptable for link blowingapplications.

FIG. 9 shows optical details of a preferred direct, unleveraged lenssystem. The input beam 91 is nominally collimated and the voice coil 23maintains the laser spot in the calculated target field as determined bythe trajectory planner 12. The objective 24 translates on an air bearingsled 26. The preferred embodiment has the advantage of a common,precision optical axis for both spot size change and rapid focusadjustment. The zoom beam expander 25 is optically compensated tomaintain the collimation with spot size change, and a small residualerror is accommodated with the dynamic focus objective 24.

The relationship between the depth of the beam waist and the position ofthe link is computed via the trajectory planner 12. Given a detailedprescription of the lens surface positions, indices of refraction thoseskilled in the art can make use of standard ray trace methods forGaussian laser beams to calculate the change in depth of the beam waistas a function of the translation of the focusing assembly.

FIGS. 8 a-8 c show in detail a preferred construction of theopto-mechanical system of the present invention in the form of anassembly drawing. A precision V-block assembly 81 is used for the z-axisdynamic focus assembly. The air bearing sleds 83 are used forpositioning both the objective lens 84 and zoom lens 85, and isadvantageous for overcoming limitations in accuracy and reliability. Theuse of hard bearings would be problematic at the fine scale of movement(within DOF tolerance—<0.1 um increments) and at the relatively highfrequency (typically 100-250 Hz). In addition to noise and reliabilityissues (ie: wearing mechanical parts) X,Y displacements during Z axismotion are much better controlled or eliminated with the air bearingsystem. Such displacements, even if a fraction of a micron, can lead tolink severing results which are incomplete (ie: contamination) orpossibly damage surrounding structures. The assembly step correspondingto 89 depicts the air bearing sleds within the v-block 81. Twoindependent voice coils 86 are used to position objective lens 84 andzoom telescope 85, with the zoom adjustment typically much lessfrequent. The diagram depicts the stators with a hollow (cut-out) regionwhich allows for transmission of the processing beam through the system.The overall assembly 81 can be adjusted with hold down magnets tocompensate for static offsets (e.g. pitch, roll).

Included with the z-axis optical assembly is a precision positionsensing system 22,23 with signals derived from a digital to analogconverter, for instance a 16 bit device. The trajectory planner, whichgenerates digital position data used by the motion system, isoperatively connected to the DAC. The position sensor may be acapacitive feedback sensor which are commonly used with precision lowinertia scanners (galvanometers) as described in Laser Beam Scanning,pp. 247-250, 1985, Marcel Dekker Inc. Other types of position sensorsmay also be used, for instance LVDTs (linear variable differentialtransformers) or precision linear encoders provided the requirements forlow noise, high stability, and reliability are met.

In typical operation the preferred system has a range of travel of about3 mm, a 3 db (small signal bandwidth) of about 250 Hz, and precision(repeatability) of about 46 nm (0.046 um). The 250 Hz bandwidthcorresponds to a small signal rise time of about 1 msec. Improvedresults could potentially be achieved, for instance with a higherprecision DAC. However, it is advantageous to operate the system in theapproximate specified range because servo performance is important, andoperation at a much higher bandwidth and associated high frequency noisecould introduce performance limitations. As a threshold for performancethis unit produces a significantly higher “speed-accuracy product” thanwhat can be provided with z (wafer) stage movement, and is well adaptedto follow a planar surface trajectory with precision within aboutone-tenth the tolerable DOF, with comparable x,y pointing stability.

It should be noted that this precision optical system 22-27 is alsoadvantageous for obtaining the reference data, thereby eliminating anyerror in registration between a “probe” beam and the “processing” beam9. In the preferred embodiment a modulator 2, typically an acousto-opticdevice or Pockels cell, is used to produce a low power beam for thefocus measurements at the reference locations using the “depth of focus”procedure earlier described.

Relative Motion and Profiles

When the three-dimensional coordinates of the laser beam and links to beprocessed are determined, a motion control program utilizes a trajectorygenerator 12 to efficiently process the target structures. Referring toFIGS. 10 a and 10 b, in a preferred system the acceleration and velocityprofiles are associated with the following “motion segment” types:

-   -   1. PVT (Position/Velocity/Time) segment 110. It is used to        accelerate to a desired position and velocity. The time required        to traverse this segment is optional; if not specified, the        minimum time is computed.    -   2. CVD (Constant-Velocity/Distance) segment 111. This type has        only a single scalar specification: path length of the segment.        The beam is to move at constant velocity for the specified        distance. The velocity is that specified by the endpoint of the        previous segment. Process control is typically executed during a        CVD segment.    -   3. CVT (Constant-Velocity/Time) segment. This is the same as the        CVD segment, but he segment's duration is specified rather than        its length.    -   4. Stop Segment 113. This segment takes no specifications—it        stops the stage as quickly as possible.

“Blast” refers to firing of the laser pulse to sever the links 114.Furthermore, a “stop” segment terminates motion, preferably as fast aspossible. Process control and link blowing are most often associatedwith the constant velocity segment.

In a preferred system acceleration and velocity profiles are used togenerate the x, y motion in cooperation with a DSP based servocontroller 16. The lens translations along the optical axis arecoordinated with the x, y motion so that the beam waist will bepositioned at the target location when the laser is pulsed. Hence, withthe present invention the z coordinate of the beam waist may bedynamically adjusted between any two structures on the wafer, includingadjacent structures arranged in a row (along X or Y direction) on asingle die. The incremental Z-axis resolution (smallest heightdifference) for link blowing is preferably about 0.1 um, for example,with about 0.05 um at the limit.

It should be noted that the reference surface may be offset by a fixedor variable level from the actual target (link) surface as a result ofdepositing layers (for instance an insulation layer) below the link (forinstance). In a preferred system a parameter or variable will beincluded which will offset the beam waist position accordingly, eitherfor a reference site or for the entire wafer, depending upon the levelof layer thickness control.

Generating the Laser Pulse

The laser-processing beam is typically provided by a Q-switched YAGlaser having a pre-determined pulse width, repetition rate, andwavelength as disclosed in U.S. Pat. No. 5,998,759. Alternatively, afiber laser using a semiconductor diode seed laser and a fiber laser maybe used to provide improved control over the temporal pulse shape,thereby allowing for processing of smaller links with less risk ofdamage to surrounding structures as described in co-pending applicationSer. No. 09/473,926, filed Dec. 28, 1999. In either case a controlsignal 20 is supplied to the laser 1 which generates a pulse incoordination with the continuous positioning of the wafer and lens.Those skilled in the art will recognize the coordination of the laserand motion will most likely be compounded by instantaneous or cumulativeposition error. In a preferred embodiment a programmable firing delay isincluded that adjusts the previously “scheduled” laser pulses tocompensate for such position errors. The time resolution of suchcorrection is preferably 25 nanoseconds or less. Preferably, thecomplete error correction is defined by a “tracking vector” whichconverts the total position error into a delay. This tracking vector canbe included with the transformation matrices 45 which are operativelyconnected to the controller for dynamically relating the coordinatesystems of FIG. 5, for instance.

CONCLUSION

A primary advantage of the method and system of the present inventioncan be summarized as a “speed-resolution product” in 3 dimensions—wherethe control system for the wafer movement preferably provides continuousmovement in 2 directions, and the high precision lens actuator providessmooth, continuous motion in the third dimension.

The alignment time is also reduced by estimating a surface that is usedto define a trajectory.

Those skilled in the art will recognize various alternatives fortrajectory planning, precision positioning of an optical system, and DSPbased servo control techniques and other embodiments. However, the scopeof the invention is to be limited only by the following claims.

1-28. (canceled)
 29. A method of precisely positioning a waist of amaterial-processing laser beam to dynamically-compensate for localvariations in height of conductive links of a semiconductive memorylocated on a plurality of dies formed on a wafer and spaced apart withina laser-processing site, the laser beam having a spot diameter, W(z),variable along an optical axis, and a minimum spot diameter, Wo, at abeam waist location, the method comprising: controllably positioning thewaist of the laser beam along the optical axis to dynamically adjust thebeam waist location between first and second links to be processedwithin the site so that: a spot diameter, W(z), at a processing locationis no more than about 5% greater than a minimum beam waist diameter overa total distance of about 1.5 microns or finer along the axis, and at arate fast enough so that throughput is not substantially effected by thestep of controllably positioning.
 30. The method of claim 29 wherein thestep of controllably positioning is carried out at a rate fast enough sothat a link processing speed within the site which is a function ofpulse generation rate and wafer velocity is not limited by the step ofcontrollably positioning and wherein the wafer velocity is greater thanabout 50 mm/second.
 31. The method of claim 29, wherein the step ofcontrollably positioning is carried out at a rate corresponding to abandwidth greater than 100 Hz.
 32. The method of claim 31, wherein therate corresponds to a bandwidth greater than 150 Hz.
 33. The method ofclaim 29, wherein the minimum beam waist diameter is about 1.7 micronsor less.
 34. The method of claim 33, wherein the minimum beam waistdiameter is about 1.4 microns or less.
 35. The method of claim 29,wherein the step of controllably positioning is based on a sensedposition of an optical element along the optical axis.
 36. The method ofclaim 35, wherein sensed position is provided by a capacitive sensor.37. The method of claim 29, wherein the conductive links have a pitchless than about 3 um.
 38. The method of claim 37, wherein the conductivelinks have a pitch less than about 2 um.
 39. The method of claim 29,wherein the laser beam has a wavelength of about 1.047 microns.
 40. Themethod of claim 29, wherein the total distance along the optical axis isabout 1 micron or finer.
 41. The method of claim 29, wherein the step ofcontrollably positioning is within about one-tenth of the totaldistance.
 42. The method of claim 30, wherein the link processing speedcorresponds to maximum velocity of the wafer in the range of about50-150 mm/sec.
 43. The method of claim 29, wherein the step ofcontrollably positioning controls time to move the beam waist betweenthe links to within about 0.03 msec or less.
 44. A method for preciselypositioning a waist of a material-processing laser beam to dynamicallycompensate for local variations in height of spaced apart conductivelinks of a semiconductive memory within a laser-processing site, thelinks lying on a surface which is substantially orthogonal to an opticalaxis, the method comprising: controllably positioning the waist of thelaser beam along the optical axis to dynamically adjust the beam waistlocation between first and second links to be processed within the siteso that: a spot diameter W(z) at a processing location is no more thanabout 5% greater than the minimum beam waist diameter over a totaldistance of about 1.5 microns or finer along the axis and at a rate fastenough so that throughput is not substantially effected by the step ofcontrollably positioning.
 45. A method of laser processing of conductivelinks of a semiconductive memory, the method comprising: planning atrajectory to position a laser beam waist relative to the links to beprocessed; and controllably positioning the beam waist along an opticalaxis based on the trajectory to dynamically adjust the beam waistlocation between first and second links to be processed so that: a spotdiameter, W(z), at a processing location is no more than about 5%greater than a minimum beam waist diameter over a total distance ofabout 1.5 microns or finer along the axis and at a rate fast enough sothat throughput is not substantially effected by the step ofcontrollably positioning.